Download Simple calibration for ceramic sensing elements using an ME651

AMSYS
Simple calibration for
ceramic sensing elements
using an ME651
This article describes in detail a simple electronic circuit based on a suitable
IC and few external components to amplify and to calibrate the offset- and
fullspan-signal of ceramic sensing elements. The aim is to create a ready-tomount sensor system with a ratiometric output signal and a suitable level of
precision without a temperature compensation.
Sensor system
(ME651 and AM457)
In the example application described
here a monolithic ME651 ceramic
resistive pressure measurement cell [1]
with a thick-film resistance Wheatstone
bridge is used as a sensing element
(see Figure 1). The pressure range is
2bar, gage and bridge resistance is
specified as being 10kΩ and the supply
voltage is set to 5V.
The example is not fixed in the ME651,
it is applicably to all ceramic sensing
elements.
Figure 1: Ceramic sensing element ME651
In order to be able to amplify and to calibrate the offset and span of the sensing element a
simple network was developed using the high-precision amplifier AM457 from Analog
Microelectronics in Mainz, Germany (see Figure 2). The whole signal conditioning circuit
consists of the ceramic sensing element, the integrated circuit AM457, also 4 resistors and
a minimum of 2 capacitors (see the AM457 data sheet [2]).
The AM457 IC is used as a ratiometric amplifier to convert the ME651 output signal from
ca. 10mVFS to an output signal of 4.5V. The zero point value is 0,5V and the full scale
signal is 4,5V.
The electronic components were able to be mounted on a round printed board (Ø 14mm)
which was adapted to correspond to the assembly guidelines for the ceramic sensing
element.
Using the components listed above a compact module was developed which is hereinafter
referred to as the sensor system. This system can be mounted onto a package whose
proportions are given on the AMSYS website.
Page 1/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
R4
C2
AM457
R1
7
VCC 8
OP
OUTOP
1k
9k
VCC
C4
3
C6
R3
Ceramic
sensing
element
2
6
Diagnosis
IN+
IN-
Temp
AMP
DIAG 5
OUT
INOP
4
VCC=5V
C5
GND
VDIAG
1
C1
VOUT
GND
Figure 2: Circuit for the signal conditioning of the ceramic sensing element
ME651 using the AM457 amplifier IC
Dimensioning
The circuit can be dimensioned with the aid of formulae (1) to (3). These are calculated
using a model which is based on the classic Wheatstone bridge circuit of four resistors.
These three simple equations allow us to calibrate the system with a minimum of data
pertinent to the sensing element.
The formulae used are:
2 RB
R1 R2
R4
R3
5 d
R1
100
8 d
8 d
RS
(kΩ)
(kΩ)
RO
d
RO
9 d
RS
R4
RS
(kΩ)
Span adjustment
(1)
Auxiliary resistor
(2)
Zero point adjustment (3)
whereby the following applies to the individual sensing elements:
RB – Bridge resistance in ohms
dRS – Span/supply voltage in mV/V
dRO – Offset/supply voltage in mV/V
Page 2/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
In order to calculate the discrete adjusting resistors the offset, span and bridge resistance
of the sensing element to be calibrated are first determined at room temperature and a
supply voltage of 5V. The sensing element is then connected up to an AM457 to form the
circuit illustrated in Figure 2. Resistors R1 to R4 are dimensioned according to the given
formulae. The values of the capacitors are selected according to Table 1.
Name
Value
C1
1nF
C2
100pF
C3
3.3nF
C4
1nF
C5
100nF
C6
470pF
Table 1: Capacitor values
Notes
Optional
Optional
Optional
Optional
Using the calculated resistances the zero point UFP1 and the full scale signal UFS1 are now
set with a good precision for a sensing element with a four resistors Wheaststone bridge.
As standard ceramic sensing elements have extra resistors in addition to the four bridge
resistors there is a discrepancy between the model and the sensing element,
demonstrated by the fact that the zero point UFP1 and the full scale signal UFS1 do not yet
match the setpoint. They should thus be treated as approximations.
This discrepancy is accounted for in the calibration procedure described in the following by
correcting the discrete adjusting resistors.
The obtained zero point value UFP1 is corrected to the target value of UFP2 = 0.5V by
trimming resistor R3 (e.g. potentiometer). As the zero point- and full-scale signal are
dependent in a linear correlation, when the offset is adjusted the full-scale signal also
changes to the value UFS2. So that this value can be corrected the relevant span resistors
R1 and R2 must then be adjusted. As the given formulae designate linear functions, these
resistors can simply be corrected to the obtained full-scale value of UFS2 using the ratio
Target(Full-scale)Value USOLL, whereby the circuit is now set.
Dimensioning example
1. Determining the offset, span and bridge resistance of the ceramic sensing
element
The measurement results of the sensing element are:
Offset (0 bars) = -0.096mV
FS (2 bars) = +11.53mV
Span
= +11.63mV
RB
= +10.56kΩ
Page 3/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
2. Calculating dRO and dRS
dRO = Offset/5V
dRS = Span/5V
= -0.019mV/V
= +2.33mV/V
3. Dimensioning resistors R1, R2, R3 and R4
Dimensioning the span resistors according to Formula (1):
2 RB
R1 R2
= 1.81MΩ
5 d
RS
According to Formula (2) auxiliary resistor R4 is calculated as follows:
R1
R4
= 18.1kΩ
100
Dimensioning offset resistor R3 according to Formula (3):
8 d R O d RS R 4
R3
= 2.17kΩ
8d
9 d
RO
RS
NB: Discrepancies between the setpoint value and the theoretical value enter the offset
error proportionally. It is thus recommended that the resistors are partitioned as graded
series resistors.
With the real resistors R1 = R2 = 1.81MΩ, R3 = 2.17kΩ and R4 = 18.18 kΩ the following
measurement values are obtained for the sensor system:
PMIN1 (0 bars)
PMAX1 (2 bars)
Æ
Æ
UFP1 = 0.70V
UFS1 = 4.48V
4. Correcting the resistances of resistors R1, R2, R3 and R4
Looking at these values we can see that the zero point value UFP1 for P = 0 bars is ca. 40%
too high with regard to the setpoint of 0.5V; this is down to the discrepancy between the
model and the actual resistor structure of the ceramic sensing element. The next step in
the process is thus to adjust offset UFP2 for P = 0 to the setpoint by trimming the resistance
of R3. In the given example the value UFP2 = 0.5V has been set using R3,KORR = 1.2kΩ.
Page 4/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
Measurement values for P = 0 and P = 2 bars after trimming R3 to UOFF2 = 0.5V:
PMIN2 (0 bars)
PMAX2 (2 bars)
Æ
Æ
UFP2 = 0.50 V
UFS2 = 4.28 V
Due to the linear dependency of the zero point offset and full scale signal in the network
developed here UFS changes by the same amount as UFP. For production purposes this
means that the value PMAX2 (pressure measurement) no longer has to be measured.
As the correlations are linear, resistors R1 and R2 can also be corrected linearly.
Calculating the XFS correction factor for resistors R1 and R2:
U
4.5V
X FS= FS ,SOLL =
= 1.05
U FS 2
4.28V
Calculating the new corrected values for R1 and R2:
2 RB
R 1 , KOR R R 2 , KOR R
X FS = 1.90MΩ
5d
RS
Measuring the new calibrated UFS,KAL voltage with corrected resistances R1,KORR and
R2,KORR:
PMIN2 (0 bar)
PMAX2 (2 bar)
→
→
UFP2
= 0.493V
UFS,KAL = 4.468V
Calibration errors:
Zero point error:
0.493V – 0.5V = -0.007V
which is equivalent to 0.16%FS
Full scale error:
4.468V – 4.5V = -0.032V
which is equivalent to 0.71%FS
Page 5/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
Measurements
The sensor system compensated for using corrected resistors R1, R2, R3 and R4 was
characterized with regard to pressure and temperature. Figure 3 gives the output signal
and its dependency on the applied pressure at TR and Vcc = 5V.
VOUT/[V]
2bar /
4.468V
4.5
3.5
2.5
0bar /
0.493V
1.5
0.5
p/[bar]
0.5
0
1.0
1.5
2.0
Figure 3: Output of the sensor system (AM457+ME651)
dependant of the applied pressure at T = 25°C
Using 1ppm/K resistors the zero point and the full scale voltage of the sensor system were
measured for the industrial temperature range.
0,56
Zero point voltage of the ME651 + AM457 system
dependent on the temperature at P=0bar
0,55
0,54
0,53
VZero point [V]
0,52
0,51
0,50
0,49
0,48
0,47
0,46
0,45
0,44
-40 -30 -20 -10
0
10
20
30
40
50
60
70
80
90
100
Temperature [ªC]
Figure 4: Temperature drift of the zero point voltage at the system output
Page 6/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
4,70
Full-scale voltage of the ME651 + AM457 system
dependent on the temperature at P=2bar
4,65
4,60
VFULLSCALE [V]
4,55
4,50
4,45
4,40
4,35
4,30
-40 -30 -20 -10
0
10
20
30
40
50
60
70
80
90
100
Temperature [°C]
Figure 5: Temperature drift of the full scale voltage at the system output
With a non-selected sensing element an zero point temperature drift of 0.004%FS/K and a
full scale temperature drift of 0.02%FS/K was determined at the system output.
This example deliberately refrains from compensating for the temperature coefficients of
the zero point (TCOS) and full scale (TCSS) of the sensor system. The reason for this lies
in the fact that calibrating temperature at system level is time-consuming and expensive;
the level of accuracy achieved is usually sufficient for most applications without
temperature compensation.
With regard to the system the primary temperature errors are the TCO and TCS of the
sensing element. With its excellent temperature behavior IC AM457's contribution to the
temperature error is marginal.
Even if no compensation for temperature is necessary, the temperature drift of the
adjusting resistors, which directly affects the TCOS and the TCSS, should nevertheless be
taken into consideration. For this reason it is recommended that resistors with a suitable
temperature coefficient are used.
Should the achieved accuracy of temperature not be sufficient, both the TCO and TCS of
the sensing element can be calibrated by the manufacturer.
Page 7/8
Simple calibration for ceramic
sensing elements using an ME651
AMSYS
Conclusion
The given example illustrates how a ceramic sensing element was calibrated using an
AM457 and some discrete resistor components. It was possible to determine the
necessary resistances by making just one single iteration.
The suggested calibration method using discrete resistors is suitable and has a sufficient
accuracy for all ceramic sensing elements
• which permit calibration with resistors
• which have an output signal of ±5mVFS…±100mV FS
• where the span signal is greater than the offset.
AM457's diagnostic unit was not used in this example; it can be applied without change of
the presented results.
Further reading
http://www.amsys.de/
[1] The ME651 data sheet
http://www.analogmicro.de/
[2] The AM457 data sheet
Page 8/8